1. Field of the Invention
The present invention relates to an active material used for an electrode of a secondary battery.
2. Description of the Related Art
In recent years, with an increase of environmental engineering, development of power generating technologies which pose less burden on the environment (e.g., solar power generation) than conventional power generation methods has been actively conducted. Concurrently with the development of power generation technology, development of power storage technology has also been underway.
A power storage technology includes, for instance, a lithium ion secondary battery. Lithium ion secondary batteries are widely prevalent since their energy density is high and because they are well suited for miniaturization. As an active material used for a positive electrode of the lithium ion secondary battery, there is olivine structure LiFePO4, for example.
Olivine structure LiFePO4 (lithium iron phosphate) has favorable characteristics since the lithium atoms (Li) are arranged in one direction without being inhibited by other atoms. However, since Li is a rare metal, its reserves are few and it is expensive. Therefore, sodium (Na), which is plentiful at low cost, is being considered as a substitute material for Li.
Conventional NaMPO4 (M is Mn, Fe, Co or Ni) takes a maricite structure (for reference, see Patent Document 1 and Patent Document 2). In the maricite structure, since the sodium atoms contributing to electrical conduction are not arranged in one direction without being inhibited by other atoms, the drift of the applied field effect state of sodium ions (Na ions) is small, thus there is the problem that favorable characteristics can not be obtained.